The present invention is directed toward controlled curing of devices requiring optical cure. More specifically, the present invention provides a method for curing optical devices such that the devices undergo a more controlled polymerization, resulting in a reduction in defects such as dimpling and warpage in the cured device. In particular, the optical devices include ophthalmic lenses including contact lenses, intraocular lenses, spectacle lenses, corneal onlays and corneal inlays. More particularly, this method provides for a method to produce contact lenses having a controlled cure profile.
It is often desirable to mold optical devices such as contact lenses and intraocular lenses, rather than form the lenses by machining operations. In general, molded lenses are formed by depositing a curable liquid such as a polymerizable monomer into a mold cavity, curing the liquid into a solid state, opening the mold cavity and removing the lens. In particular, the mold cavity may be formed by a mold assembly comprised of a posterior mold portion and an anterior mold portion, each having a lens-forming surface. When the posterior mold portion and anterior mold portion are mated, the lens-forming surface of the posterior mold portion and the lens-forming surface of the anterior mold portion form the lens-forming cavity. The non-lens-forming surface of both mold portions, herein referred to as non-critical surfaces, are generally molded to have a similar radius (or radii) of curvature as that of the lens-forming surfaces. While the lens-forming surfaces are of optical quality, each having a central optical zone and typically, at least a peripheral carrier zone, the only requirement of the non-critical surface generally is a smooth surface.
Polymerization is typically carried out by thermal means, irradiation or combinations thereof Traditionally, conventional thermo-casting techniques require fairly long curing times and are used when the resultant object is thick. Rods from which rigid gas permeable lenses are lathed from or thicker lenses are often thermally cured. Curing of lenses by irradiation, in particular, ultraviolet (UV) irradiation, frequently offers short curing times. The monomer is poured into a transparent mold having a desired optical surface, and thereafter the UV light is radiated to the monomer through the transparent mold to cure the photosetting monomer.
A common material used as a mold material is polypropylene, which is disclosed in U.S. Pat. No. 5,271,875 (Appleton et al., assigned to Bausch and Lomb Incorporated, the entire contents herein incorporated by reference). The process disclosed in Appleton et al., may be used to produce lenses with predictable and repeatable characteristics.
The use of polypropylene may be desired with certain lens-forming materials. Other lens-forming materials, however, may cast just as well or better in other mold materials. As disclosed in U.S. Ser. No. 09/312105 (Ruscio et al. and assigned to Bausch and Lomb Incorporated, the entire contents herein incorporated by reference), polyvinyl chloride absent any UV stabilizer provides a suitable material for the posterior mold.
While the irradiation of the optical device from the light source may be conducted in a uniform and parallel manner, the material chosen for the mold portions may affect the pathways of the light rays. For instance, some materials, such as thermoplastic crystalline polymers, may diffuse the radiation, causing a scattering of the light rays. Polypropylene is such a material. Other materials such as polyvinyl chloride and polystyrene are thermoplastic amorphous polymers, which permit an unhindered pathway for the light rays during curing.
The radiation may also be reflected off the surface of the glass or plastic mold materials. This may result in non-uniform distribution of light intensity over the lens-forming material.
This invention recognized that the non-critical surface of the posterior mold may act as an optical device, reflecting and/or refracting the irradiation in a non-uniform pathway through the mold portion. In particular, the geometry of the non-critical surface of a posterior mold may affect the cure of the lens article sandwiched between the posterior and anterior mold. For instance, the non-critical surface of a posterior mold may be comprised of a radius of curvature and an outer flat portion. The junction formed at the intersection of the radius and flat portion may produce a molding area in which the radiation does not penetrate well. This would be similar to providing a shadow on the lens article. The resultant lens would then have a circular area corresponding to the junction(s) that may not be as completely cured as the rest of the lens. Any junction formed at the intersection of different geometries may produce xe2x80x9cshadowingxe2x80x9d. The geometries need not necessarily be spherical.
Additionally, the non-critical surface of the mold may refract the radiation from the optical source. This may lead to non-uniform curing speed of the ultraviolet curable resin. As a result, since the curing is completed faster and more completely in a portion receiving a high radiation intensity (in this instance, the periphery portion of the lens) and slower in a portion receiving a low irradiation intensity (the central portion, respectively), a stress is generated in the cured resin layer. This stress may deteriorate the precision of the optical device face. Additionally, since the faster curable portion receiving higher radiation intensity is cured with absorption of the surrounding uncured resin in order to compensate for the contraction of resin resulting from curing, the slower curable portion (which receives lower radiation intensity) may show defects such as shrinkage. In particular, in the case of contact lenses and spectacle lenses, this can produce lenses with unacceptable optical aberrations caused by uneven curing and stress. xe2x80x9cDimplingxe2x80x9d or warpage of the contact lens is a common problem caused by uneven curing. In dimpling, the apex of the lens is flattened or slightly concave in shape. Warpage is generally seen as the inability of the edge of a lens to have continuous contact with the molding surface upon which it contacts. Other drawbacks seen with plastic spectacle lenses include xe2x80x9cstriationsxe2x80x9d, which are caused by uneven curing and stress. Thermal gradients form in the gel-state, which produce convection lines (xe2x80x9cstriationsxe2x80x9d) that become frozen in place and cannot be dispersed.
U.S. Pat. No. 4,166,088 (Neefe) discloses a method of controlling the polymerization of cast optical (plastic or contact) lenses. The mold section on the bottom is a lens which focuses UV light to the center of the cavity. The bottom mold must have a thickness which corresponds to the focal length of the refractive surface so that the UV light rays converge at the center of the monomer being cured. Neefe also requires an aluminum reflector on the outer surface of the top mold to reflect light back through the monomer.
U.S. Pat. No. 4,534,915 (Neefe) discloses the use of a convex positive refractive power cylinder lens to provide a band of actinic light to a rotating lens monomer. The center of the spin cast lens receives the most radiation, the area adjacent to the center receives less while the periphery receives still less radiation. This allows for the outer portion of the spin cast lens to migrate inward as the lens shrinks during the curing process. A fresnel lens or a Maddox rod may also be used to provide the narrow high-energy line of actinic light.
U.S. Pat. No. 4,879,318 (Lipscomb et al.) discloses the use of mold members formed from any suitable material that will permit UV light rays to pass through. To aid in the even distribution of the UV light, the surfaces of the molds are frosted. In one embodiment, a Pyrex glass plate is used to filter out UV light below a certain wavelength. Lipscomb et al. found that if incident UV light is not uniform throughout the lens, visible distortion pattern may appear in the finished lens. Lipscomb et al. solved this problem by including additives in the lens forming composition to reduce the distortions. The ophthalmic lenses are formed from plastic.
U.S. Patent No. 4,919,850 (Blum et al.) discloses a method for making plastic lenses in which the liquid lens material is dispensed into the mold cavity and put into a heated bath for a partial thermal curing. After a period of time, the mold (while still in the liquid bath) is subjected to UV light for an additional period of time. The liquid bath disperses the UV light sufficiently to avoid stresses and other adverse effects on the lens ultimately formed that may be caused by uneven exposure to the UV light. The mold may also be rotated while in the bath or the bath may include an aerator to enhance the dispersion of the UV rays. By rotation of the mold and aeration of the bath, the surface of the mold is also kept free of any debris which may otherwise channel the UV light. Additionally, a reflective surface provided on the one of the molds forms may reflect UV light back through the lens material being cured.
U.S. Patent No. 4,988,274 (Kenmochi) discloses irradiating the central portion of the mold cavity containing the lens-forming material to initiate a photocuring reaction. The area of the light, in the shape of a ring, is enlarged until the lighted area reaches the periphery of the lens-forming material. A variable power lens, including a fresnel lens, may be used to align the light. The lens-forming material in the center of the mold cavity is cured first which causes the lens-forming material around it to shrink. The shrunk volume of lens-forming material is supplemented with additional uncured lens-forming material. The variable power lens allows for adjustment of the ring-shaped light.
U.S. Pat. No. 5,135,685 (Masuhara et al.) discloses the use of a conveyor or other moving device to continuously move objects to be irradiated by a multiplicity of aligned sources of visible light. The movement of the irradiated objects may be linear or curved movement on the same plane or upward or downward movement.
U.S. Pat. No. 5,269,867 (Arai) discloses a method for producing glass lenses with a coating on one side. The coating is a resin layer that is cured with UV light. The resin is dropped onto a metal mold (with a reflective surface) and the glass lens placed on the resin. The resin is interposed between the lens and the metal mold. UV light is provided through the glass lens, curing the resin. A filter may be used to evenly distribute the UV light. Without the filter, the reflection of the metal mold and the glass lens result in non-uniform distribution of UV light and non-uniform curing speed. The center of the resin cures faster than the outer perimeter, causing defects such as shrinkage in the resin.
U.S. Pat. No. 5,529,728 (Buazza et al.,) discloses a method of curing a plastic eyeglass lens. The method comprises placing a liquid polymerizable composition within a mold cavity defined by mold members and a gasket. A first set of UV rays is directed to one of the mold members. The gasket is removed and a second set of UV rays is directed to the lens. Buazza et al., further discloses the use of a filter which includes a plate of Pyrex glass to diffuse the UV light so that it has no sharp intensity discontinuities. To produce a positive lens, the UV light intensity is reduced at the edge portion so that the thicker center portion of the lens polymerizes faster than the thinner edge of the lens. Mold members of Buazza et al., are preferably precision ground glass optical surfaces having UV light transmission characteristics including casting surfaces with no surface aberrations, waves, scratches or other defects.
None of the above art completely solves the problems which occur when using a mold assembly in which one mold portion is made from an amorphous material and acts as an optical device. The resultant lens made from this particular mold assembly may have defects such as dimpling and warpage.
The present invention is a method for photocuring cast articles such as ophthalmic lenses in which defects in the cured article are reduced. By altering the pathway by which irradiation rays reach the article to be cured, defects can be reduced. By controlling the relative intensity of radiation upon a particular portion of lens-forming material, the rate of polymerization taking place at various portions of the lens can be controlled. This method is particularly suited for use with mold materials which are amorphous.
One embodiment of this invention comprises altering the radiation rays by at least partially neutralizing the optical affects of the non-critical surface through which the rays initially penetrate. This can be accomplished by filling the non-critical surface cavity of the posterior mold with a liquid, such as water or glycerin.
Another embodiment of this invention comprises reshaping or removing any junctions formed between different geometries (e.g. a radius of curvature and flat surface portion) used to form the non-critical surface. The non-critical surface of the posterior mold may then be comprised of a controlled radius of curvature, eliminating any shadows or areas which the irradiation rays may not penetrate evenly. The controlled radius may be spherical or aspherical, provided that the surface is smooth and continuous.
In still another embodiment of this invention, the radiation path from a light source is altered so as to obtain a desired cure profile across the mold cavity. This results in controlled cure gradient across the cast article. The radiation path may be altered in various ways, including the use of an optical element. The optical element may be a positive or negative lens.
In the preferred embodiment, the optical element is a positive lens which is placed at a predetermined distance above the posterior mold. The positive lens converges radiation rays, preferably ultraviolet (UV) radiation, passing through the mold and increases the energy available to the cured article. The distribution of irradiation rays radiates from the center of the mold. This distribution reduces the cure gradient across the lens, which reduces or removes any residual stress induced during curing. The result is a cured article such as a contact lens having an acceptable apex in the central portion of the lens. The positive optical element allows control of the illumination intensity profile reaching various sections of the contact lens. Stress developed by uneven intensity profiles can be removed or introduced.
It is further preferred that the radiation path is altered by use of an aspheric condenser lens such that the light rays passing through the posterior lens mold is distributed radially. The aspheric condensing lens is placed at a certain distance from the mold and preferably has a plano back. The lens may be any lens that focuses light to a certain desired area.
An alternate method of altering the radiation path is placement of an optical element into the non-critical surface cavity of the posterior mold. In particular, the radiation can be altered by placement of a lens in the shape of a plug into the non-critical surface cavity of the posterior mold. Preferably the lens is a solid asymmetric convex lens and the material from which it is formed is amorphous.
The ophthalmic lenses formed from these methods are relatively free from defects such as dimpling and warpage.